ESTIMATION OF MOMENTUM AND HEAT FLUXES USING DISSIPATION AND FLUX-VARIANCE METHODS IN THE UNSTABLE SURFACE-LAYER

Dissipation and flux-variance methods, derived from the turbulent kine tic energy and temperature variance budget equations in conjunction wi th Monin-Obukov similarity theory, were used to estimate surface fluxe s of momentum and sensible heat. To examine the performance of these t wo methods, direct eddy correlation measurements were carried out abov e a nonuniform grass-covered forest clearing in Durham, North Carolina . The dissipation method sensible heat flux predictions were iii good agreement with eddy correlation measurements. Also, the flux-variance method reproduced the measured sensible heat flux well following an ad justment to the similarity constant. However, the momentum flux (or fr iction velocity) estimated by the dissipation and flux-variance method s were both inferior to those for sensible heat flux. The data from th is experiment indicated that the above two methods are sensitive to th e dimensionless wind shear (phi(m)) and temperature standard deviation (phi(theta)) functions. On the basis of dimensional analysis and the temperature variance budget equation a new dissipation approach for es timating sensible heat flux was derived. The similarity constant for t his new approach was shown to be around 1.6 for uniform surfaces and f rom the data of this experiment.